Air bag systems have been developed for automotive use. The air bag systems can quickly inflate an air bag with a gas generated from a gas generating composition to prevent the riders from being injured or killed by crashing, through inertia, against a steering wheel, a front windshield or other solid or hazardous parts in the automobile or the like in the event of collision of the automobile or the like running at a high speed. Gas generating compositions suitable for use with automotive air bag systems are to meet rigorous requirements. First, the air bag is required to be inflated in a very short time, usually within 30 to 50 milliseconds. Further, an optimum atmosphere in the bag corresponds in the composition to the air in the automobile or the like.
Currently available gas generating compositions containing an alkali metal salt or alkaline earth metal salt of hydrazoic acid, particularly sodium azide, as a gas generating base are able to meet the foregoing requirements and are good. However, these gas generating compositions have the drawback that the sodium azide as the main component and alkali components produced as by-products in the generation of a gas are toxic. Fears are entertained as to the environmental pollution entailed in the mass disposal of air bag system-loaded automobiles.
To overcome the foregoing problem, azide-free gas generating compositions have been developed as a substitute for sodium azide-based gas generating compositions. For example, Japanese Unexamined Patent Publication No.208878/1991 discloses a composition comprising tetrazole, triazole or a metal salt thereof as the main component, an oxygen-containing oxidizing agent such as ammonium perchlorate, sodium nitrate, etc., and a metallic oxide such as V.sub.2 O.sub.5, CuO, Fe.sub.2 O.sub.5, etc. Generally the air bag system is adapted to remove the undesired substances from the generated gas by filtration before the discharge of the gas into the bag. The role of the metallic oxide in the disclosed composition is to form a solid product of combustion which can be easily filtered. On the other hand, Japanese Examined Patent Publications Nos.6156/1989 and 6157/1989 disclose gas generating compositions comprising a metal salt of a hydrogen-free bitetrazole compound as the main component. Further, Japanese Unexamined Patent Publication No.213687/1993 discloses a gas generating composition comprising a transition metal complex of aminoarazole as the main component. The azide-free compounds taught in the foregoing series of prior art publications have the feature that carbon monoxide is produced in a low concentration because the compound has a small number of carbon atoms per molecule. But the disclosed compositions are all unsatisfactory in the time required for inflating the bag.
The inventor of the present invention previously found that an azide-free gas generating composition comprising, as active components, a specific nitrogen-containing organic compound such as azodicarbonamide and a specific oxygen-containing inorganic oxidizing agent such as potassium perchlorate is unlikely to cause environmental pollution and fully satisfactory in the bag-inflating time and is advantageous also in terms of costs. Then, the inventor filed patent applications on these findings (Japanese Unexamined Patent Publications Nos.32689/1994, 32690/1994 and 227884/1994). Such gas generating compositions produce remarkable results which are as follows.
(a) These compositions generate a large amount of a gas per unit mass, thereby contributing to the miniaturization of automotive air bag inflators and to the reduction of their weight. Forty grams of the gas generating composition suffices for a 60 l air bag in contrast with 60 to 80 g of conventional gas generating compositions required for the same bag.
(b) The compositions have the combustibility required of inflators. In a 60 l tank test, the compositions are equivalent to conventional gas generating compositions in the results plotted in a time/pressure curve.
(c) The compositions are prepared from low-toxicity compounds and thus are of lower toxicity themselves than conventional compositions.
(d) The compositions are prepared from low-hygroscopicity compounds and thus are of low hygroscopicity themselves. Thus, they can be more easily handled than conventional compositions.
(e) The gas and the suspended particulate substances produced by the combustion of the gas generating composition are relatively low in toxicity and lower in toxicity than those from conventional compositions. Accordingly the solid components of combustion product can be substantially completely removed by the filter used in conventional inflators.
(f) Even if an air bag-loaded automobile or transport vehicle carrying inflators should fall into water, spreading the composition in the water, a less damage would be done by the pollution than the case of conventional compositions.
(g) After the operation of an air bag, copper and potassium chloride predominantly remain as trapped by the filter in the inflator. These substances are unlikely to adversely affect the operators' health during the disassembly of air bag systems.
It is well known that in burning a carbon-containing organic compound, carbon monoxide is produced as an incompletely burned substance even if an oxidizing agent is used in an amount sufficient to generate an amount of oxygen essentially required for burning the carbon, hydrogen and combustible elements in the organic compound, namely even if it is used in excess of a stoichiometric amount. Consequently the nitrogen-containing organic compound such as azodicarbonamide which is a base for the gas generating composition is expected to produce a relatively large amount of carbon monoxide as a by-product on combustion of the compound particularly because of a great number of carbon atoms present per molecule of the compound.
An attempt may be made to use a catalyst useful in the conversion from carbon monoxide to carbon dioxide in order to avoid the production of carbon monoxide as a by-product. Numerous compounds are known as the catalyst as disclosed, for example, in "List 1 of Classification of Catalysts According to Reactions" (edited by Tarama Laboratory, Kyoto University, published by Kagaku Kogyo Sha, pp.291-292). But there is unknown a catalyst which can meet the reaction conditions of gas generating compositions for an air bag, that is, can exhibit an effective reactivity within a contact time of generally approximately tens of milliseconds.